The present invention describes an apparatus and method for determining the energy expenditure of a subject by indirect calorimetry where respiratory gases O 2 and CO 2 are captured in a tent mask coupled through a pneumotach flow meter to a vacuum inlet of a blower fan whose speed is controlled by a computer running a program that ensures that the bias flow being drawn through the mask always exceeds the subject's peak expiratory flow. VO 2 and VCO 2 and flow values are measured on a breath-by-breath basis and used in arriving at a bias flow adjustment voltage to be applied to the blower fan for adjusting its speed.